1. Field of the Invention
This invention relates to asphaltic based mastic materials particularly useful in vibration damping applications.
2. Description of the Prior Art
Important reductions in automobile noise level have been achieved by vibration damping of automobile floor panels, door panels, and roof panel sections. Compositions containing fillers dispersed in binders have been widely used to coat such metal surfaces to provide the desired sound deadening. For example, sheet materials of heat softenable compositions including asphalt, fillers and natural and/or synthetic elastomers have been used in this application. Heat softenable sheet materials of this type are described in detail in U.S. Pat. No. 4,133,932, Australian Pat. No. 498,074 and Canadian Pat. No. 997,085.
Sheet materials as described above are presently used in the automotive industry for sound deadening of metal floor panel sections. Essentially, the sheet material is placed on the floor panel section before the automobile body goes through the heating ovens for drying the paint finish. In response to the temperatures of the oven, the sheet material softens (or wilts) into conformity with the contour of the floor panel section and adheres to the metal surface.
The automotive industry has established tests for acceptable sound deadening sheet materials. These tests require that the sheet materials soften into conformity with the contours of the panel section surface at the paint oven temperatures and provide a strong bond to the metal surface. At the same time, the softening of the sheet material must be controlled or restrained so that flowing or running of the thermoplastic material not exceed an established value at the oven temperatures or at higher temperatures. Particularly desired sound deadening sheet materials for floor panel sections are those having the capability of bonding directly to the metal surface without the need of applying adhesive layers to the metal and/or the sheet material surfaces.
Another desirable performance characteristic of sound deadening sheet materials is that they be non-blocking or not stick together when stored in layers. The non-blocking feature permits the sheet materials to be stacked together during storage and transportation without the use of a release sheet between adjacent sheets. Sound deadening sheet materials requiring release sheets impose obvious disadvantages for an assembly line operation in terms of time, labor and costs.
More recently, the automotive industry has been especially concerned with weight reduction in order to improve fuel efficiency. To this end, expandable or foamed, non-asphaltic, sound deadening sheet materials of reduced density have been considered. However, proposed foamed sound deadening sheet materials are relatively expensive and sound deadening sheet materials have been traditionally assigned a low cost status in the automotive industry. The increased expense is due primarily to the high concentration of polymer in the sheet material required to provide a foamed sheet material of acceptable functional integrity. An acceptable foamed sound deadening sheet material must provide a cellular structure which is sufficiently strong and resilient to sustain continuously applied pressures without breaking or permanently collapsing.
A proposed bitumen containing foamable sound deadening material is described in German Ausligeschrift No. 2,824,681. The material is described as a non-self supporting mass foamable to the porous foamed structure comprising bitumen. Apparently the described material is designed to flow or run at elevated temperatures since the material is applied in the form of a foil 23 mm thick, reduced to a thickness of 11.5 mm presumably by flowing and then expanded or foamed to provide a porous structure. Additionally, the Example describes the foamable mass as containing about 25 percent by weight bitumen mixed with about 75 percent by weight mineral filler and a heat activatable adhesive is used to bond the mass to the metal.
The present invention is directed to the problem of providing a realtively inexpensive sound deadening sheet material which meets the desired flow characteristics at elevated temperatures, has the desirable surface adherent capability but is nonblocking and most importantly, can be expanded to provide a cellular structure of desirable and acceptable strength and integrity.